1. Field of the Invention
The present invention relates to a magnetic device and a frequency analyzer.
2. Related Background Art
A conventionally known magnetoresistive element is a GMR (Giant Magnetoresistive) element in which a nonmagnetic electroconductive layer is interposed between a fixed layer with the direction of magnetization fixed, and a free layer with the direction of magnetization freely varying. Another known magnetoresistive element is a TMR (Tunnel Magnetoresistive) element in which a nonmagnetic insulating layer is interposed between the fixed layer and the free layer. When an electric current is allowed to flow through these magnetoresistive elements, a spin-polarized electric current flows to generate a torque by virtue of interaction with spins accumulated in the free layer, and the direction of magnetization of the free layer changes according to the polarities of the spin-polarized electric current. In a state where the free layer is located in a constant magnetic field and even in a case where the direction of magnetization is subject to change, the torque acts on the direction of magnetization so as to return it to the stable direction restrained by the magnetic field. The motion of the direction of magnetization is similar to swinging oscillation of a pendulum weight pulled by gravity and swayed by a specific force, and is called precession.
It was recently found that a resonance phenomenon occurs when the frequency of the precession of the direction of magnetization coincides with the frequency of the alternating current flowing through the free layer (cf. Non-patent Document 1). The magnetic resistance of the TMR element is determined by the difference between the direction of magnetization of the free layer and the direction of magnetization of the fixed layer. When the direction of magnetization resonates in the free layer, the direction of magnetization of the free layer largely oscillates and the magnetic resistance largely varies periodically. On the other hand, when the magnetic resistance largely varies in synchronization with the input alternating current, the alternating current flowing between the two ends of the TMR element asymmetrically varies with respect to the zero level and comes to have a direct current component, and the variation can be extracted as an output.
In order to cause the above-described phenomenon in the TMR element, it is necessary to apply a large magnetic field to the TMR element, and it is common practice to locate the TMR element in a magnetic-field applying device and perform experiments in that state. [Non-patent Document 1] Nature, Vol. 438, 17 Nov. 2005, pp. 339-342